Internet Engineering Task Force (IETF) M. Tuexen
Request for Comments: 7053 I. Ruengeler
Updates: 4960 Muenster Univ. of Appl. Sciences
Category: Standards Track R. Stewart
ISSN: 2070-1721 Adara Networks
November 2013
SACK-IMMEDIATELY Extension forthe Stream Control Transmission Protocol
Abstract
This document updates RFC 4960 by defining a method for the sender of
a DATA chunk to indicate that the corresponding Selective
Acknowledgment (SACK) chunk should be sent back immediately and
should not be delayed. It is done by specifying a bit in the DATA
chunk header, called the (I)mmediate bit, which can get set by either
the Stream Control Transmission Protocol (SCTP) implementation or the
application using an SCTP stack. Since unknown flags in chunk
headers are ignored by SCTP implementations, this extension does not
introduce any interoperability problems.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7053.
Tuexen, et al. Standards Track [Page 1]

RFC 7053 SACK-IMMEDIATELY November 2013
2. However, if the situation can only be detected by the sender of
the DATA chunk, [RFC4960] provides no method of avoiding a delay
in sending the SACK. Examples of these situations include ones
that require interaction with the application (e.g., applications
using the SCTP_SENDER_DRY_EVENT, see Section 4.1) and ones that
can be detected by the SCTP stack itself (e.g., closing the
association, hitting window limits, or resetting streams, see
Section 4.2).
To overcome the limitation described in the second case, this
document describes a simple extension of the SCTP DATA chunk by
defining a new flag, the "I bit". By setting this bit, the sender of
a DATA chunk indicates that the corresponding SACK chunk should not
be delayed.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The (I)mmediate Bit in the DATA Chunk Header
Figure 1 shows the extended DATA chunk.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | Res |I|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier | Stream Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Extended DATA chunk format
The only difference between the DATA chunk in Figure 1 and the DATA
chunk defined in [RFC4960] is the addition of the I bit in the flags
field of the DATA chunk header.
Tuexen, et al. Standards Track [Page 3]

RFC 7053 SACK-IMMEDIATELY November 2013
[RFC4960] defines the Reserved field and specifies that these bits
should be set to 0 by the sender and ignored by the receiver.
4. Use Cases
The setting of the I bit can either be triggered by the application
using SCTP or by the SCTP stack itself. The following two
subsections provide a non-exhaustive list of examples of how the I
bit may be set.
4.1. Triggering at the Application Level
One example of a situation in which it may be desirable for an
application to trigger the setting of the I bit involves the
SCTP_SENDER_DRY_EVENT in the SCTP socket API [RFC6458]. Upper layers
of SCTP that use the socket API as defined in [RFC6458] may subscribe
to the SCTP_SENDER_DRY_EVENT to be notified as soon as no user data
is outstanding. To avoid an unnecessary delay, the application can
request that the I bit be set when sending the last user message
before waiting for the event. This results in setting the I bit of
the last DATA chunk corresponding to the user message; this is
possible using the extension of the socket API described in
Section 7.
4.2. Triggering at the SCTP Level
There are also situations in which the SCTP implementation can set
the I bit without interacting with the upper layer.
If the association is in the SHUTDOWN-PENDING state, setting the I
bit reduces the number of simultaneous associations for a busy server
handling short-lived associations.
Another case is where the sending of a DATA chunk fills the
congestion or receiver window. Setting the I bit in these cases
improves the throughput of the transfer.
If an SCTP association supports the SCTP Stream Reconfiguration
extension defined in [RFC6525], the performance can be improved by
setting the I bit when there are pending reconfiguration requests
that require that there be no outstanding DATA chunks.
Tuexen, et al. Standards Track [Page 4]

RFC 7053 SACK-IMMEDIATELY November 20135. Procedures5.1. Sender-Side Considerations
Whenever the sender of a DATA chunk can benefit from the
corresponding SACK chunk being sent back without delay, the sender
MAY set the I bit in the DATA chunk header. Please note that why the
sender has set the I bit is irrelevant to the receiver.
Reasons for setting the I bit include, but are not limited to (see
Section 4 for the benefits):
o The application requests to set the I bit of the last DATA chunk
of a user message when providing the user message to the SCTP
implementation (see Section 7).
o The sender is in the SHUTDOWN-PENDING state.
o The sending of a DATA chunk fills the congestion or receiver
window.
o The sending of an Outgoing SSN Reset Request Parameter or an SSN/
TSN Reset Request Parameter is pending, if the association
supports the Stream Reconfiguration extension defined in
[RFC6525].
5.2. Receiver Side Considerations
Upon receipt of an SCTP packet containing a DATA chunk with the I bit
set, the receiver SHOULD NOT delay the sending of the corresponding
SACK chunk, i.e., the receiver SHOULD immediately respond with the
corresponding SACK chunk.
6. Interoperability Considerations
According to [RFC4960], the receiver of a DATA chunk with the I bit
set should ignore this bit when it does not support the extension
described in this document. Since the sender of the DATA chunk is
able to handle this case, there is no requirement for negotiating the
support of the feature described in this document.
7. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to set the I bit.
Please note that this section is informational only.
Tuexen, et al. Standards Track [Page 5]

RFC 7053 SACK-IMMEDIATELY November 2013
A socket API implementation based on [RFC6458] needs to be extended
to allow the application to set the I bit of the last DATA chunk when
sending each user message.
This can be done by setting a flag called SCTP_SACK_IMMEDIATELY in
the snd_flags field of the struct sctp_sndinfo structure when using
sctp_sendv() or sendmsg(). If the deprecated struct sctp_sndrcvinfo
structure is used instead when calling sctp_send(), sctp_sendx(), or
sendmsg(), the SCTP_SACK_IMMEDIATELY flag can be set in the
sinfo_flags field. When using the deprecated function
sctp_sendmsg(), the SCTP_SACK_IMMEDIATELY flag can be in the flags
parameter.
8. IANA Considerations
Following the chunk flag registration procedure defined in [RFC6096],
IANA has registered a new bit, the I bit, for the DATA chunk.
The "Chunk Flags" registry for SCTP has been updated as described in
the following table.
DATA Chunk Flags
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | E bit | [RFC4960] |
| 0x02 | B bit | [RFC4960] |
| 0x04 | U bit | [RFC4960] |
| 0x08 | I bit | [RFC7053] |
| 0x10 | Unassigned | |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
Tuexen, et al. Standards Track [Page 6]

RFC 7053 SACK-IMMEDIATELY November 20139. Security Considerations
See [RFC4960] for general security considerations for SCTP. In
addition, a malicious sender can force its peer to send packets
containing a SACK chunk for each received packet containing DATA
chunks instead of every other received packet containing DATA chunks.
This could impact the network, resulting in more packets sent on the
network, or the peer, because the generating and sending of the
packets has some processing cost. However, the additional packets
can only contain the simplest SACK chunk (no gap reports, no
duplicate TSNs), since in cases of packet drops or reordering in the
network a SACK chunk would be sent immediately anyway. Therefore,
this does not introduce a significant additional processing cost on
the receiver side. This also does not result in more traffic in the
network, because a receiver sending a SACK for every packet is
already permitted.
10. Acknowledgments
The authors wish to thank Mark Allmann, Brian Bidulock, David Black,
Anna Brunstrom, Gorry Fairhurst, Janardhan Iyengar, Kacheong Poon,
and Michael Welzl for their invaluable comments.
11. References11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Chunk Flags Registration", RFC 6096,
January 2011.
11.2. Informative References
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458, December 2011.
[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, February 2012.
Tuexen, et al. Standards Track [Page 7]